The arrest this week of two rural California men, after one acknowledged he attended an Al Qaeda camp in Pakistan to "learn to kill Americans," has revived questions about the hidden presence of potential terrorists on the home front.

To some experts, the arrests in what appears to be a widening investigation in Lodi are a sign that some homeland-security procedures are working as hoped. To others, the news suggests that Al Qaeda still has a dangerous and unknown reach into cities and towns across the US.

For now, Lodi, a farm town outside Sacramento with a longstanding Muslim community, has become a locus of concern. But beyond the ongoing investigation here in California, the case renews a broader question of how far Al Qaeda's US reach extends.

"This shows that Al Qaeda is still communicating with, recruiting, training, and sending people ... to carry out major operations worldwide, as they have steadily since 9/11," says Brian Jenkins, a terrorism expert at RAND Corp. in Santa Monica, Calif. At the same time, he says, the arrests also highlight the unprecedented coordination of intelligence services around the world to foil possible attacks.

Terror-related arrests have occurred this month in other parts of the country as well. In Florida and New York, two US citizens - Tarik Shah and Rafiq Abdus Sabir - were arrested in an FBI sting, accused of conspiring to provide material support to a terrorist organization. And in Falls Church, Va., Maher Amin Jaradat was arrested for fraudulently procuring US citizenship, with federal agents alleging he failed to disclose ties to militant groups.

In Lodi, the FBI arrested two US citizens, Hamid Hayat and his father, Umer, on charges of lying to federal agents about involvement and financing of a terror camp in Pakistan.

The men were not charged with planning specific attacks. But investigators anticipate that the ongoing probe may result in new insights about terror operations inside and outside the US.

The investigation in Lodi expanded late Wednesday to include three other local Muslims - including two imams - detained for alleged immigration violations. Investigators are trying to determine whether the arrests are evidence of a network of Al Qaeda supporters in the agricultural area.

Hamid Hayat first came to the attention of the FBI on May 29. Authorities discovered that his name was on the US government's "no fly" list while he was en route from South Korea to San Francisco. Such lists, used by the Department of Homeland Security, include names of suspected terrorists culled from confiscated computers in terrorist raids world wide.

Such procedures have been widely criticized for obstructing the movement of people with Arabic and Arabic-sounding surnames. Although the guilt or innocence of the men remains unproved, the arrests can be viewed as a "triumph, [given] all the criticism the DHS has taken over its no-fly lists," says Matthew Lippman, a professor of criminal justice at the University of Illinois. "We have not actually seen such an effectively documented case of domestic investigative effectiveness to this point."

The father's attorney, Johnny Griffin III, stressed that his client, like the son, so far has been "charged with nothing more than lying to an agent."

The two men had seemed to fit in well in the community, which to some observers raises anew the prospect of innocent Muslims arousing suspicion and fear among their neighbors.

While some experts focus primarily on what the arrests say about America's homeland-security strengths, others worry about continued weaknesses.

Among their concerns is that, despite the intensity of America's war on terror, training camps like the one mentioned by Hayat apparently still exist.

"It is really sobering to think that this far after 9/11 , there are still Americans traveling to foreign training camps to become educated on how to kill Americans," says Aitan Goelman, a federal prosecutor in the Oklahoma City bombing and a former official with the terrorism and violent crime section of the US Department of Justice.

Still, questions have emerged about the training camp Hayat described. Some Some experts say a location at Rawalpindi, near Islamabad, is implausible.

In an FBI affidavit, Hayat is quoted as saying there were "hundreds of attendees from various parts of the world at this camp," who were given choices to carry out their jihad: in the US, Afghanistan, Iraq, Kashmir, or elsewhere.

Another concern is what some see as the growing complacency of the American public, thanks in part to the lack of major terrorist attacks since 2001. By conservative estimates, there may be about 100 Al Qaeda operatives in the US. A liberal figure - encompassing those who would support terrorism as well as carry out attacks - might be in the low thousands.

"There still may be just hundreds or thousands of such operatives in the US," says Gary Bowersox, an Afghanistan and Pakistan specialist. "No one really knows, but this just shows the fact that they really are here, just waiting for direction."

Others warn against overstating the risk. "The fact that there have been no attacks since 9/11 suggests that we shouldn't overreact [and think] they have big infrastructure here," says Bruce Hoffman, a researcher at RAND's Center for Terrorism and Risk Management Policy in Washington. "Al Qaeda has become a generic term.... So are these people really that or something else? We really don't know enough of the details yet."

• Material from wire services and The Washington Post was used in this story.